<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      const str = "hello jack";
      /*
      包装成了对象，系统自动完成的
      str = {
        toUpperCase(){}
      }
      */
      console.log(typeof str);
      console.log(str.toUpperCase()); //str对象类型
      console.log(typeof str);
    </script>
  </body>
</html>
